Research Journal of Applied Sciences, Engineering and Technology 5(4): 1351-1357, 2013
ISSN: 2040-7459; e-ISSN: 2040-7467
© Maxwell Scientific Organization, 2013
Submitted: July 05, 2012
Accepted: July 31, 2012
Published: February 01, 2013
Surface Modification of Commercially Pure Titanium by Plasma Nitrocarburizing at
Different Temperatures and Duration Process
1
Agung Setyo Darmawan, 2Waluyo Adi Siswanto and 3Tjipto Sujitno
Jurusan Teknik Mesin, Universitas Muhammadiyah Surakarta (UMS), Pabelan,
Surakarta 57102, Indonesia
2
Department of Engineering Mechanics, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja,
Batu Pahat, Johor, Malaysia
3
National Nuclear Energy Agency (BATAN), Yogyakarta 55281, Indonesia
1
Abstract: One of potential metals to be used in biomechanical applications is the commercially pure (cp) titanium.
This material requires a process to improve the mechanical properties of the surface, because it is relatively soft. The
purpose of this study is to determine the effect of plasma nitro carburizing process to cp titanium surface hardness.
In this study, cp titanium plasma nitro carburizing process is conducted at different temperatures, i.e., at 350°C for 3,
4, and 5 h, and at 450°C for 2, 3, and 4 h, respectively. Hardness tests are then performed on each specimen. The
depth of penetration in the hardness test is also recorded; the microstructure captures are also taken using an optical
microscope. The results show that the longer processing time, the higher the hardness value. In higher temperature,
the hardness values correspond to the increasing temperature. In terms of the depth direction, there is a reduction in
hardness value compared to the raw material.
Keywords: Biomechanics, diffusion, hardness, surface hardening, vickers
INTRODUCTION
Titanium was first discovered in England by
W. Gregor in 1791 and named in 1794 by H. Klaproth
(Aladjem, 1973). In Mendeleef’s periodic table,
titanium is one of the transition elements in group IV
and period 4. The material has low density property
relative to other structural metals and alloys, but it has
an excellent corrosion resistance (Tan and Zhu, 2007;
Yu et al., 2005; Dong and Bell, 2000). Table 1 shows
the physical properties of unalloyed titanium.
Nowadays, titanium is widely used in industry and
medical field (Trtica et al., 2006; Fu and Batchelor,
1998; Luo and Ge, 2009; Hamada et al., 2002).
In industry, about 80% of its usage is in the field of
aerospace, besides that, the use of a pretty significant
also in the field of chemical and petrochemical (Bloyce
et al., 1994).
Due to its biocompatibility, titanium is used in
medical applications including surgical implement and
implants, such as hip balls and sockets (hip joint
replacement) that can last up to 20 years (Schank,
2012). Titanium is also used in other several medical
fields such as dental implant materials, bone fitting,
replacement of the skull, and the retaining structure of
the heart valves (Liu et al., 2004a; Liu et al., 2004b;
Tian et al., 2005; Shenhar et al., 1999). Because
titanium is a non-ferromagnetic material, the implant
Table 1: Physical properties of unalloyed titanium (Liu et al., 2004a)
Property
Value
Atomic number
22
Atomic weight (g/mol)
47.90
Crystal structure
Alpha, hexagonal, closely packed
C(Å)
4.6832±0.0004
a(Å)
2.9504±0.0004
Beta, cubic, body centered
a(Å)
3.28±0.003
Density (g/Cm3)
4.54
8.4×10-6
Coefficient of thermal expansion, 𝛼𝛼, at
-1
20°C (K )
Thermal conductivity (W/(mK))
19.2
Melting temperature (oC)
1668
Boiling temperature (estimated) (oC)
3260
o
Transformation temperature ( C)
882.5
Electrical resistivity
High purity (μΩCm)
42
Commercial purity (μΩCm)
55
105
Modulus of elasticity, 𝛼𝛼, (GPa)
692
Yield strength, 𝛼𝛼, (MPa)
785
Ultimate strength, 𝛼𝛼, (MPa)
to the patient is very safe with magnetic resonance
imaging.
Available titanium which is commonly used in the
hip joint replacement is Ti-6Al-4V Titanium Alloy.
When wearing is one of parameters that mostly
considered, the Titanium alloy has a good wear
resistance. But, there are some concerns about the
toxicity of Al and V wear debris in the human body that
might negative side effects to the human body. For this
Corresponding Author: Agung Setyo Darmawan, Jurusan Teknik Mesin, Universitas Muhammadiyah Surakarta (UMS),
Pabelan, Surakarta 57102, Indonesia
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Res. J. Appl. Sci. Eng. Technol., 5(4): 1351-1357, 2013
reason, the use of cp titanium is a potential metal and
environment, more economical, and, lower gas
safer to replace Ti-6Al-4V Titanium Alloy bearings.
consumption compared to other nitro carburizing
However, when cp titanium is used as a part of
techniques (Karakan et al., 2004).
which will experience friction with other parts such as
In this study, the influences of plasma nitro
that is in hip joint replacement, then the addition of the
carburizing to hardness of cp titanium’s surface are
hardness value will be required (Ali et al., 2011;
investigated. Hardness, one of mechanical property that
Darmawan et al., 2010). However when the hardness is
important to consider, is a measure of a material’s
increased by a certain treatment to the whole part, there
resistance to localized plastic deformation (e.g., a small
is a possibility the material will become brittle, which is
dent or a scratch). Hardness is an indicator of wear
not expected. Any excessive loading applied to the
resistance and ductility. The instrument used to measure
material will damage it easily.
the hardness is Micro Vickers. Micro Vickers is
Increasing the hardness at the surface without
referred to as micro indentation-testing method on the
changing the ductility properties of materials on the
basis of indenter size. It is well suited for measuring the
inside of the material will increase the toughness
hardness of small, selected specimen regions (Callister,
properties of material without changing the ductility if
2007). Hardness test is very useful for materials
the inner part. This process is referred to as surface
evaluation, quality control of manufacturing processes,
hardening.
and development effort.
Several surface hardening techniques can be used
to increase hardness of titanium and its alloy such as:
MATERIALS AND METHODS
Ion implantation (Ali et al., 2011; Huang et al., 2004;
Jagielski et al., 2006) plasma spraying (Liu et al.,
The material used for this work is cp titanium. The
2004b; Miklaszewski et al., 2011), laser beam (Tian
chemical composition of cp titanium is as follows: N:
et al., 2005; Grenier et al., 1997), vacuum (Kim et al.,
0.04%, C: 0.05%, H: 0.003%, Fe: 0.13%, O: 0.11%, Al:
2009), and powder immersion reaction (Shenhar et al.,
0.49% S: 0.03, Ti: balance. Micro structure photograph
1999).
of this material can be seen in Fig. 1 the capture shows
Diffusion methods of surface hardening modify the
that the material consists of 100% α-phase and that
chemical composition of the surface with hardening
there is no β-phase present. α-phase has a unit cell of
species such as nitrogen, carbon, or boron. These
hexagonal close packed (hcp) and β-phase has body
methods allow effective hardening of the entire surface
center cubic (bcc).
of a part and are generally used when a large number of
Regarding to plasma nitro carburizing processes,
parts are to be surface hardened. One of the diffusion
cp titanium material is cut with the size of 1×1×0.3 Cm.
methods is Nitro carburizing.
Specimens as many as 6 pieces created for this purpose.
Nitro carburizing is a thermo chemical process in
Three specimens used in the process of plasma nitro
which a process of diffusion of nitrogen and carbon
carburizing at 350°C while the other 3 are used at
atoms toward the surface of metallic materials at certain
450°C. Then the material is grinded and polished using
elevated temperatures. Heat is needed to enhance the
polycrystalline diamond until it is clean and shiny.
diffusion of hardening species into the material’s
In plasma nitro carburizing process, the plasma is
surface. Nitro carburizing can be applied to liquid,
formed in a vacuum by means of high voltage electrical
solid, and plasma atmospheres. Nitro carburizing
energy in which the positive ions of nitrogen, carbon
widely used to increase the hardness value in the
and hydrogen are accelerated to strike the cathode. The
surface of steel (Krishnaraj et al., 1998; Bell et al.,
work piece is maintained at a negative dc high voltage
2000).
source of 250-850 volts in the presence of an electric
Jones (2012) defined that plasma is different with
field. The gases are separated, ionized, and accelerated
traditional phase of matter such solids, liquids, and
toward the work piece (cathode). The kinetic energy of
gases. Plasma is a collection of charged particles that
ion is converted into heat energy by ion bombardment.
respond strongly and collectively to electromagnetic
This energy not only heating the work piece but also
fields, taking the form of gas-like clouds or ion beams.
implanting ions directly and resulting the cathode
Since the particles in plasma are electrically charged
(generally by being stripped of electrons), it is
frequently described as an ionized gas.
It was Irving Langmuir who assigned the term
"plasma". Langmuir and his colleague, Albert W. Hull,
contributed a joint paper and used the term plasma on
grid-controlled gas tubes to the National Academy of
Science in 1929 (Brittain, 2010).
The plasma nitro carburizing produces faster
Fig. 1: Microstructure of cp titanium
nitrogen and carbon diffusion, more friendly with
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Fig. 2: Schematic diagram of devices for plasma nitro carburizing
Fig. 3: Photograph of the device arrangement for plasma nitro
carburizing
sputtering. The electrons are forced out from the
surface of the work piece. Some of the ions implanted
into the surface of the specimen, the other led to the
cathode sputtering. Furthermore, absorption and
diffusion of nitrogen and carbon atoms led to the
formation of the compound layer. The schematic of
equipment setup and photograph of the device
Fig. 4: Vickers hardness test
arrangement for plasma nitro carburizing can be seen in
Fig. 2 and 3, respectively.
After the plasma nitro carburizing process
Plasma nitro carburizing processes are carried out
completed, each specimen is tested of its hardness using
in 2 elevated temperature i.e., 350 and 450oC,
a micro vickers hardness tester.
respectively. In the plasma nitro carburizing at 350°C, 3
work pieces are tested in different duration processes;
Prior to hardness testing, a careful surface
3, 4, and 5 h respectively. In another experiment at
preparation (grinding and polishing) is conducted to
temperature of 450oC, the nitro carburizing process of 3
ensure a well-defined indentation that may be
accurately measured. This hardness test follows the
samples are hold in different duration; 2, 3, and 4 h,
standard ASTM E 384. The micro-hardness
respectively.
measurement works with indenter force as light as 10 gf
For the plasma nitro carburizing process with
(gram force), with indentation time in 15 sec. The
temperature of 350°C, the pressure condition is
indenter is a square-based pyramidal-shaped diamond
maintained at 1.6 mbar. The electrical charge for the
with a face angle of 136°C. This indenter and its
process is at 592 volt, with the current on 249 mA. At
diagonals of impression are illustrated in Fig. 4. After
the temperature of 450°C, the voltage is set higher at
force removal, the impression diagonals are measured
745 volt, with the current 357 mA.
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Fig. 6: Hardness testing locations
Fig. 5: Karl frank GMBH type 38505 buehler micro hardness
tester
with a light microscope. It is assumed that the
indentation does not undergo elastic recovery after
force removal.
The Vickers hardness number can be determined
by the following equation:
𝐻𝐻𝑉𝑉 = 1,854 𝑝𝑝/𝑑𝑑2
(1)
where,
P : The applied force (Kg)
d : Mean diagonal of impression (mm)
Karl Frank GMBH Type 38505 micro hardness
tester is used for measuring the hardness of the
specimen. This micro hardness tester can be seen in
Fig. 5.
RESULTS AND DISCUSSION
The results are analyzed and plotted to see the
effect of plasma nitro carburizing process time and
temperature process to the material hardness and to see
the hardness trend in to the variation of depth direction.
Effect of process time and temperature to material
hardness: Hardness testing is conducted at 3 locations
on the surface of the specimen, namely the location A,
B and C. Figure 6 shows these locations. After hardness
testing, the mean diagonal of impression is obtained
and then it is used to calculate the vickers hardness. The
result can be seen in Table 2 and 3.
As an example calculation, the data obtained from
plasma nitro carburizing process at temperatures of
350oC with a time of 3 h at the location A is
calculated according to Eq. (1). Then the hardness, HV,
is obtained as follows:
HV = 1,854 P / d 2
= 1,854 x0.01 / 0.50 2
= 74.16
Table 2: Process plasma nitro carburizing at temperature 350 oC
Duration
(h)
Location d 1 (mm) d 2 (mm) d (mm) HV
3
A
0.50
0.50
0.50
74.16
B
0.50
0.50
0.50
74.16
C
0.50
0.50
0.50
74.16
4
A
0.45
0.45
0.45
91.56
B
0.45
0.45
0.45
91.56
C
0.44
0.45
0.45
93.62
5
A
0.45
0.45
0.45
92.93
B
0.43
0.43
0.43
98.73
C
0.45
0.45
0.45
91.56
Table 3: Process plasma nitro carburizing at temperature 450oC
Duration
(h)
Location
d 1 (mm) d 2 (mm) d (mm)
HV
2
A
0.40
0.40
0.40
115.88
B
0.40
0.40
0.40
115.88
C
0.48
0.48
0.48
79.360
3
A
0.40
0.40
0.40
115.88
B
0.38
0.38
0.38
126.17
C
0.39
0.39
0.39
121.89
4
A
0.38
0.38
0.38
126.17
B
0.38
0.38
0.38
126.17
C
0.38
0.38
0.38
126.17
HV average
74.16
92.25
94.41
HV average
103.70
121.31
126.17
Similarly, the hardness at location B and C is
obtained. The hardness at location B is 74.16 and the
hardness at location C is 74.16.
Average hardness of locations A, B, and C is the
following:
HV average =
=
HV A + HV B + HVC
3
74.16 + 74.16 + 74.16
3
= 74.16
Furthermore, the average hardness is used as the
hardness property from each specimen.
The hardness values for the plasma specimens nitro
carburizing processes at temperatures of 350°C for
process duration of 3, 4, and 5 h are 74.16, 92.25 and
94.41 HV, respectively.
There is a significant increased in hardness value
when the process duration is extended from 3 h to 4 h.
The hardness can be improved as high as 24.4%.
The hardness improvement, however does not
show a linear behavior when the process time is
extended. When the process time is extended to 5 h, the
hardness value is increased only 2.3%. The hardness
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140
Hardness (HV)
120
100
80
60
40
Temperature 450 oC
20
Temperature 350 oC
0
0
1
2
3
Time (h)
4
3
4
Fig. 7: The hardness values in the surface of specimen which
is processed plasma nitro carburizing at temperatures
350 and 450°C
Fig. 8: Ti (N, C) compound layer which is resulted by using
plasma nitro carburizing process at temperatures
350oC for 4 h
Fig. 9: Ti (N, C) compound layer which is resulted by using
plasma nitro carburizing process at temperatures
450oC for 4 h
improvement is not significant when it is compared
with that of process time extension from 3 to 4 h. The
hardness values are illustrated in Fig. 7.
Figure 7 shows that hardness value of specimens
which are resulted from the plasma nitro carburizing
process at temperature of 450°C is higher compared
with specimens that are processed at temperature of
350°C. Specimens are processed at temperature of
450°C for 3 and 4 h show hardness values of 121.31
and HV 126.17, respectively. While, those are
processed at temperature of 350°C have hardness
values of 74.16 and 92.25 HV for the process time 3
and 4 h.
The results of hardness test showed that the higher
the temperature, the hardness value will be higher, and
the longer the test time, the higher the hardness value.
This is because the diffusion of carbon and nitrogen
atoms depends on the time and temperature. The higher
the time and temperature, the more diffusion of carbon
and nitrogen atoms and the distance is longer as well.
Furthermore, the layer compound Ti (N, C) which is
formed also thicker (Fig. 8 and 9) so the hardness
become higher.
Hardness analysis in depth direction: Hardness test
results in various depth directions are shown in Fig. 10.
The hardness of the specimens which is processed by
using plasma nitro carburizing at temperatures of 450oC
has a value as high as 126.17 HV on the surface. The
hardness values in the depth directions of 0.61, 0.92 and
1.5 mm are recorded at the level of 63.49, 53.47 and
54.16 HV, respectively.
The hardness values of the specimens after plasma
nitro carburising process at temperature of 350°C are
the following: on the surface 92.25 HV, in depth
directions of 0.30, 0.6 and 1 mm are as high as 56.29,
54.85, and 53.47 HV, respectively.
For specimens which are processed by using
plasma nitro carburizing at temperatures of 450oC, the
hardness values at depth of 0.92 and 1.5 mm are
recorded at the level of 53.47 and 54.16 HV. After the
treatment process at temperature 350oC, hardness
values at depth of 0.6 and 1 mm are as high as 54.85,
and 53.47 HV. Comparing with the results at higher
temperature (450oC), the results do not show a
significant difference. The average hardness of the raw
material without any treatment is 53.99 HV.
Figure 10 also shows that the surface hardness
value of specimen which is processed plasma nitro
carburizing at temperatures of 450oC for 4 h increased
by 134%. The increase was higher than the hardness
value of specimen which is processed plasma nitro
carburizing at temperatures of 350oC for 4 h, which
only increased by 71%. These results show that the
hardness on the surface can be significantly improved
from the initial hardness level of untreated material
53.99 HV.
The plasma carburizing treatment at higher
temperature does not significantly change the hardness
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140
Hardness (HV)
120
100
•
80
60
40
Temperature 450 oC
20
Temperature 350 oC
0
0
0.2
0.4
0.8
0.6
1.0
Depth (mm)
1.2
1.4
1.6
ACKNOWLEDGMENT
Fig.10: The hardness values in the depth direction of
specimen which is processed plasma nitro
carburizing at temperatures 350 and 450oC for 4 h
properties of the material below the surface. The
treatment at both temperatures of 350 and 450°C can
significantly improve the hardness on the surface, while
inside the material the hardness remains unchanged.
This indicates that the plasma nitro carburizing has a
significant effect on the surface which is favorable. In
biomechanical applications it is intended that the
surface treatment to increase the surface hardness but
maintaining the mechanical properties of the material.
Since the hardness of the material below the surface is
not change the ductility of the material will not be
changed.
CONCLUSION
In this research, the surface hardness of cp titanium
is modified and improved by using plasma nit
carburizing process. The effects of such processes to
hardness value can be concluded as the followings:
•
•
•
•
the distance is longer as well. Furthermore, the
layer compound Ti (N, C) which is formed also
thicker so the hardness become higher.
The plasma nitro carburizing of cp titanium can
significantly increase the hardness on the surface
but does not have significant effect below the
surface. This shows that this treatment is suitable
for improving the hardness of the surface while
maintaining the original properties of the cp
titanium.
The hardness value of cp titanium can be increased
by extending the time during the plasma nitro
carburizing process. The increasing hardness rate
however does not show a linear behavior as shown
at temperature of 350 and 450°C.
The hardness value resulted from the plasma nitro
carburizing process at temperature 450°C is higher
compared with that at temperature 350°C.
The surface hardness value of specimen which is
processed plasma nitro carburizing at temperatures
of 350oC for 4 h increased by 71% while those
processed at temperatures of 450oC for 3 h
increased by 125%. However, there is no
significant increasing of hardness after extending
the processing time.
The higher the time and temperature of process, the
more diffusion of carbon and nitrogen atoms and
The authors gratefully acknowledge financial
support from Universiti Tun Hussein Onn Malaysia
under Fund Scheme GIS Vot 0809. The authors also
would like to thank Accelerator Application Group of
BATAN-Yogyakarta Indonesia for allowing plasma
nitro carburizing processes be conducted in its
laboratory; Material Laboratory of Universitas Gajah
Mada Indonesia for microvickers hardness test and
Mechanical Engineering Laboratory of Universitas
Muhammadiyah Surakarta for all specimens
preparation.
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